Mostly modernize base::Value usage in chromecast/crash/linux/

* Replace ListValue with Value::List, DictionaryValue with Value::Dict.
* Make `dump` a Value::List and metadata a Value::Dict almost everywhere,
to improve type safety.
* Use optionals instead of unique_ptrs in most places, since Values are
now generally passed around by value instead of by unique_ptr, and
optional works better with that in the cases where they can be null.
* Pass non-null values by reference instead of as pointers.

This CL leaves DumpInfo::ParseEntry() alone, since it will likely need
a helper method, and this CL is already fairly large.

Bug: 1187001, 1338341
Change-Id: Id63e1942db55ce70c7cb0d97708773579c868f8a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3781499
Reviewed-by: Sean Topping <seantopping@chromium.org>
Commit-Queue: Matt Menke <mmenke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1027859}